1. Field of the Invention
This invention relates generally to valves for controlling the flow rate of a fluid in a conduit and more specifically to butterfly valves for use with fluids which experience significant changes in temperature.
2. Description of the Prior Art
Butterfly valves have been used extensively to control the flow rate of a fluid in a conduit or pipe. Such valves commonly include a housing which defines the flow passageway for the fluid and which has in cross section the configuration of a circle. A stem or shaft has been pivotally mounted on the housing to extend along the diameter of the circle. Damper means such as a circular disk have been disposed interiorly of the housing in a substantially fixed relationship with the stem. The pivoting of this stem has tilted the damper means to open and close the passageway and thereby control the flow rate of the fluid.
The disk has been attached to the stem by configuring the stem and the disk to define at least one keyway. Keys have been disposed in the keyways so that the relative movement between the disk and the stem has been inhibited. The keyways and the keys have typically been provided along lines substantially parallel to the longitudinal dimension of the stem.
This type of valve has had a particularly high failure rate when used for controlling the flow rates of fluids which undergo significant changes in temperature. This failure rate is primarily attributed to the different temperature ranges which are experienced by the stem and the disk. The disk is more directly exposed to the fluid and is exposed to a greater quantity of the fluid than the stem. As a consequence, the temperature of the fluid is substantially that experienced by the disk. The stem, which is exposed to a lesser quantity of fluid and is often isolated from the direct flow of the fluid, undergoes a different temperature change than the disk. As a consequence, even where the disk and the stem have had the same coefficient of thermal expansion, the elongation of the respective elements at their interface has been different.
Since the keys have maintained the respective elements in a substantially fixed relationship, the different elongation characteristics of the disk and the stem have resulted in buckling and warping of the disk with the consequent overstressing of parts. These characteristics have also produced a mechanical looseness which has caused the disk to flutter when the valve has been partially opened. This, of course, has resulted in a significant wear on the elements and an ultimate failure of the valve.